Tyres known in the art have a multilayer structure, some components of which are schematically exemplified in FIG. 1. The latter shows a transverse section of the tyre, in particular a radial section taken along a plane passing through—or parallel to—the rotation axis.
With reference to FIG. 1, the most external part of the tyre comprises a toroidal tread 500 and two side walls 501 e 502, the latter arranged, in use, in a position proximal and distal to the vehicle, respectively. Bilaterally to tread 500, between it and each side wall 501, 502, transition zones (shoulders) 511 and 512 are provided.
Still in the most external part of the vehicle, in the portion close to/in contact with the rim (herein denoted by C) the tyre has abrasion gum strips 503 and 504.
The part of the tyre facing onto, and in contact with, the inner tube is the so-called inner liner 515.
The tyre main body, without tread, is generally referred to as casing or body 505.
Inside the tyre, between tread and casing, a toroidal body ply 510 is provided. Typically, a tread belt 509 separates said body ply from tread.
Each one of the two toroidal tyre portions intended to engage the rim is called bead and denoted by 506 and 507, respectively, in FIG. 1. The most external part of each bead is made of one of said abrasion gum strips 503 and 504. Inside each bead, a bundle of metallic wires (bead bundle) is provided, tipically enclosed into an elastomer casing. Said bundle, in FIG. 1, is denoted, by way of example, by 511 for bead 506. What will be said hereinafter for the latter is to be considered valid also for other bead 507.
Bead bundle 511 is associated with a bead filler 512. Such filler is typically made of an elastomer of great hardness, capable of increasing the stiffness of the tyre sides. At each bead, body ply 510 envelops the bead bundle 511 and its respective bead filler 512, so as to form a turn-up 514.
During tyre construction, each filler, which has substantially the shape of a triangular body, is coupled with the corresponding bead bundle. The two filler-bundle assemblies are laid onto the body ply, which is lying about a construction drum before the turning (folding) operation that generates the turn-ups.
Each bead filler serves the primary function of ensuring a continuous variation of the elastic return from the materials constituting the bead—which are relatively more rigid—to the respective sidewall of the tyre, relatively more elastic. However, the presence of the filler as described above induces an increase in mass and volume of the bead, and therefore in the whole tyre, that may decrease rolling efficiency.
To overcome the just mentioned disadvantage, WO 2012/098500, to the same applicant, discloses a tyre wherein each bead filler is obtained from an elastomer strip interposed between the bead bundle and the body ply. Such elastomer strip, in the body ply turn-up operation, is folded as a “U”, so as to form a ring around the bead bundle. The tyre is then cured, so that there is no need for a pre-assembly of the fillers with the respective bead bundles.
In any case, the known configurations can be optimized in terms of balance between the overall stiffness of the tyre bead, the tyre mechanical durability and the desired reduction of mass and volume.
Another critical aspect in the tyres of the known art is represented by the formation of cracks at the body ply transverse ending folded towards the tyre external part (body ply ending), sometimes indicated as point “P1” (in cross section) and so identified in FIG. 1. Such cracks generation is made easier by the friction which may arise between the body ply flaps which contact at the turn-up, and/or by the presence of an interface between materials with different mechanical characteristics, in the specific case the body ply material and the elastomeric material which constitutes the tyre outer part. The generation and subsequent rapid propagation of a crack through the respective abrasion strip limits tyre lifetime.
The disadvantage just mentioned is particularly marked in the case of so-called “low turn-up” tyres, that is, with a turn-up of limited height.
In the attempt to solve such disadvantage, the application of local reinforcements, eventually multilayer ones, has been proposed, which reinforcements are arranged at each body ply side end (i.e. on body ply endings). However, often such configurations cause a significant increase in complexity of the tyre construction method. Furthermore, none of the known solutions results in an increase of durability of such a magnitude as to compensate for the above-mentioned increase in constructive complexity.
EP 0 393 966 discloses a radial tyre having a hard rubber sheet disposed at the outer side of each turn-up portion and a soft rubber material interposed between the turn-up portion and the hard rubber sheet.
U.S. Pat. No. 4,842,033 discloses a tyre having, at the bead portions, gum strips encapsulating the cut edge of a cord reinforcement ply.